1. Field of the Invention
The present invention relates to the field of semiconductor lithography, and, more particularly, to optical pellicles to protect photomasks from particulate contamination.
2. Description of Related Art
In the semiconductor chip industry it is well known that pattern transfer from a photomask (or mask) to a substrate is accomplished by exposing a photomask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the photomask are projected onto the substrate which has been treated with a photo-sensitive substance. This results in the mask openings in the mask forming the pattern being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a frame holding a thin membrane known as a pellicle is mounted above the mask surface, such that the pellicle membrane extends parallel to the mask at a predetermined distance spaced away from it. Any contamination which would ordinarily land on the mask surface instead falls on the pellicle membrane.
Pellicles substantially eliminate the above exposure problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The pellicle typically comprises a polymer membrane approximately one micrometer thick that is stretched over a frame which is affixed to a mask, also referred to as a “reticle.” Although the pellicle is in the optical path, the polymer thickness is typically small since imaging performance degradation generally increases with increasing pellicle thickness.
Pellicles serve as dust covers of reticles and protect the increasingly more expensive reticles from being contaminated in the lithographic process. Usually, a pellicle is an assembly comprising a thin pellicle membrane layer attached to a pellicle mount frame. Because the pellicle layer has a thin thickness compared to a relatively large surface area, the layer is usually called a membrane or a film. Typically, the pellicle layer has been made by stretching a thin (1 μm) polymer layer over an aluminum frame. Aside from other concerns such a pellicle does not appear viable for new 157 nm and shorter wavelength applications because the polymers rapidly degrade under the exposure of these wavelengths.
Many pellicle membranes are fabricated of nitrocellulose or cellulose acetate, while deep UV pellicle membranes are commonly fabricated of a fluoropolymer such as Cytop® from Asahi Glass or Teflon® AF fluorocarbon amorphous polymer from DuPont. Cytop is a poly-perfluoro polymer containing a cyclic ether functional group, such as poly-perfluoro cyclo oxyaliphatic polymer. Typically a solution of the polymer is used, such as an 8% solution, and the solution is spun onto a substrate, the solvent is baked off, and the membrane is removed from the substrate in a peeling operation.
The membrane may also be formulated of Teflon AF amorphous fluoropolymer from DuPont. Teflon AF fluorocarbon is a family of amorphous copolymers of per-fluoro (2,2-dimethyl-1,3 dioxole)(PDD) and tetrafluoroethylene. Teflon AF fluorocarbon is typically based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole with fluorine-containing monomers. A solution of Teflon AF amorphous fluoropolymer is dissolved in a solvent such as Fluorinert FC-75 from 3M and spin coated to form the membrane.
An alternative was proposed by SEMATECH to use a hard pellicle comprised of silica or doped silica. Because silica has a higher modulus and density than polymer layers, initial efforts have focused on very thick (300-800 μm) silica plates to avoid sag, which causes wavefront distortion. Because of the large optical thickness of these hard pellicles, very tight specifications on absorption, flatness, bow, and wavefront distortion have been set by SEMATECH to minimize the impact on performance of the projection lithographic system.
Current pellicles however fail at a high Numerical Aperture (NA). At low angles of incidence, the pellicles are commercially acceptable but at large angles the pellicle fails to transmit all of the light. The light lost from imperfect transmission is reflected and causes undesirable ghost images. The amount of reflective loss is dependent on both the incident angle and the polarization of the incident light. State-of-the-art immersion exposure tools with a NA>1 are particularly prone to such problems and prior art pellicle structures cause significant image degradation. It is these angle-dependent reflections which are the main problem addressed by the present invention.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an optical pellicle which has enhanced transparency and operational properties.
Another object of the present invention is to provide a method for making an optical pellicle.
A further object of the present invention is to provide a method for imaging a substrate such as a semiconductor using the pellicle of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.